A.John Curran, owner of RSI General Contractors, a roofing, siding and insulation company in Syracuse, N.Y., responds: In our experience, the most effective way to insulate the counterweight cavity is to use blown cellulose. After the stops, sashes, and pulleys have been removed, we tape over the pulley hole, then drill a single 1-inch-diameter hole in the center of the jamb (where it will be covered by the new window). We then pump in cellulose until it backs up and we know the cavity is full.

The advantage of cellulose is that we can pack the cavity tight without losing R-value, as with fiberglass, or risking damage from expansion, as with expanding spray-foam. The disadvantage is that we have to lug a blower machine to the job. Fortunately, we have a small one-bag unit, which is relatively portable. These machines can be rented or...

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